The present invention relates to a flame retardant polymer compositions and more particularly to flame retardant polypropylene compositions. Such compositions are suitable for manufacturing extruded sheets useful for construction and building materials.
Polypropylene resin is widely used in the production of etching tanks, electroplating tanks, hot air ducts, computer cabinets, electrical appliances, household interior decorations, and automobile parts among many other items. It is a polymer of choice due to its good processing characteristics, chemical resistance, weathering resistance, electrical properties and mechanical strength. One major disadvantage is that polypropylene is naturally flammable. This has generated a growing demand for flame retardant type of polypropylene and many such flame retardant polypropylene compositions have been suggested in the art. For example, it is known in the art to impart flame resistance to polypropylene compositions by the inclusion of certain fillers in a molten blend of the polymer. In this regard, U.S. Pat. No. 5,130,357 forms a blend of polypropylene, a synthetic rubber, silane coupling agent, ammonium polyphosphite, and a nitrogen containing organic compound. U.S. Pat. Nos. 3,892,815 and 4,005,156 uses poly (polyhalonorbornenylalkyl) benzene blended with polypropylene. U.S. Pat. No. 5,124,404 mixes polypropylene with a copolymer of polypropylene and a brominated styrene monomer. U.S. Pat. No. 5,216,059 discloses a flame retarding additive composition, useful in thermoplastic polyolefins comprising a mixture of a halogenated bisphenol derivative and the copolymer of a halogenated vinyl aromatic grafted onto polyolefin. The preferred bisphenol derivative is tetrabromobisphenol A bis(dibromopropyl ether), and the preferred graft copolymer comprises bromostyrene grafted onto isotactic or syndiotactic polypropylene. U.S. Pat. No. 3,687,890 teaches a thermoplastic composition which may include polypropylene, calcium sulfite, calcium sulfate and calcium carbonate. Calcium sulfate fillers are also taught in U.S. Pat. Nos. 3,198,596; 3,033,731; 3,953,565; 3,976,612; and 3,981,840, among many others. Antimony trioxide as a flame retardant is taught in U.S. Pat. No. 4,962,148. U.S. Pat. No. 4,038,237 teaches a composition of an alpha olefin, antimony trioxide and calcium sulfate in the presence of an extending oil, chlorinated hydrocarbon and a block copolymer of an elastomer and a non-elastomer. U.S. Pat. No. 3,803,067 imparts fire retardant properties to polypropylene with antimony trioxide, calcium sulfate and chloroparaffins, among other fillers.
It is most desirable to form a polypropylene composition which can be used for the formation of very smooth, lightweight sheets which have a sufficient degree of flame retardancy to meet the requirements of construction grade materials. In this regard, the flame retardant polypropylene compositions of the prior art suffer from a number of disadvantages. In order to satisfy the flame retardancy requirements for commercial use, the material must pass the Underwriters Laboratory Test Subject 94 which is described hereinafter. The materials described above either are incapable of meeting this requirement or, in order to meet the test specification must use an inordinate amount of filler with the result that sheets formed from the material are unacceptably dense. In addition, the large amount of filler produces a sheet material which is insufficiently smooth and at best require an additional, expensive, pressing step after sheet extrusion in order to produce a smooth sheet.
The present invention provides a composition which meets these requirements. That is, a polypropylene composition is provided which is capable of forming extruded sheets which meet the UL 94 flame retardancy test specification and is a low density material having a specific gravity of less than 1.1, preferably less than 1.0. It furthermore exits from a sheet forming extruder with a very smooth surface which does not need to undergo an additional pressing surface step to attain such a smooth surface. The polypropylene product of the present invention is advantageous since it has a lower cost and lower density than known flame retardant polypropylene materials while retaining its usefulness for the production of such items as tanks, ducts, cabinets, appliances, automobile parts and the like. In addition, the composition of this invention does not produce irritating fumes during manufacture.